1. Field of the Invention
This invention relates generally to an apparatus and method for cooling a plastic film tube. More particularly, this invention relates to an apparatus and method for cooling a molten plastic film tube exiting the outlet of an extrusion die in a blown film process by using evaporative cooling.
2. Description of the Related Art
Thin plastic film may be produced, in addition to other manufacturing methods, by extruding plastic material, such as polyethylene resin, in a process known as blown film extrusion. In blown film extrusion, polyethylene resin is fed into an extruder where an extrusion screw pushes the resin through the extruder. The extrusion screw compresses the polyethylene, heating the resin into a molten state under high pressure. The molten, pressurized polyethylene is fed through a blown film extrusion die having an annular opening. As the molten material is pushed into and through the extrusion die, a plastic film tube emerges from the outlet of the extrusion die.
The plastic film tube is blown or expanded to a larger diameter by providing a volume of air within the interior of the plastic film tube. The combination of the volume of air and the plastic film tube is commonly referred to as a bubble between the extrusion die and a set of nip rollers. The plastic film tube is commonly cooled by one or more external air rings applying a constant flow of air upward along the outside of the plastic film tube. A number of factors including, but not limited to, the air pressure within the bubble of the plastic film tube, the cooling rate provided by the air ring, the temperature and flow of material out of the extrusion die, and the rate at which the plastic film tube is pulled by the nip rollers impact the blow-up ratio, diameter of the plastic film tube, and the ultimate thickness or gauge of the plastic film tube.
As the plastic film tube cools travelling upward toward the nip rollers, the plastic film tube solidifies from a molten state to a solid state after it expands to its final diameter and thickness. The point along the bubble where the plastic film solidifies is known as the frost line. Consequently, the portion of the bubble below the frost line is molten allowing for expansion and thinning of the plastic film tube. Conversely, the portion of the bubble above the frost line has solidified and the diameter and thickness of the plastic film tube is generally fixed at that point.
Improved cooling of the plastic film increases the stability of the bubble, thereby allowing for more accurate control of the physical properties of the plastic film tube and for a broader range of plastic film tube diameters and thicknesses. To facilitate better cooling, it is known in the art to use an internal bubble cooling, or IBC, assembly to dissipate heat from within the interior of the bubble. Without an IBC assembly, the bubble contains a static volume of air applying outward pressure on the plastic film tube but there is no avenue to dissipate the heat absorbed by the static volume of air from the plastic film tube. Consequently, the interior air volume quickly settles at approximately the same temperature as the plastic film tube, providing no cooling benefit. Internal bubble cooling assemblies exchange the warm air within the bubble with cooler air while maintaining a constant pressure. The cooled air within the bubble absorbs heat from the interior surface of the bubble, cooling the plastic film tube more quickly and lowering the frost line for increased bubble stability.
U.S. Pat. No. 7,753,666 issued to Greg Wood and is entitled Apparatus and Method for Cooling Plastic Film Tube in Blown Film Process (hereafter, “the Wood patent”). The Wood patent describes an improved internal bubble cooling assembly using particular air ring assemblies. The internal air ring assemblies improve the flow of the cooled air within the bubble to provide improved bubble stability and allow for improved properties of the plastic film tube. The teachings and specification of the Wood patent are incorporated herein by reference.
Extruded tubular film, even with improved cooling, still has variations in the gauge or thickness of the film around the circumference from variations in the extrusion die. Consequently, the thickness of the film is typically reflected as an average thickness, or more precisely, a mean thickness based upon several measurements. However, even for films of a particular mean thickness, it is desirable to minimize the variations in thickness around the circumference of the tubular film as much as possible during the manufacturing process. By reducing variations in film thickness, there are fewer thin or weak areas on the film, which improve the overall strength and performance of the film since the thin regions represent common points of failure for plastic film products. There are several methods known in the art for achieving more uniform thickness around the circumference of the tubular film.
U.S. Pat. No. 4,339,404 issued to Hartmut Upmeier et al. and is entitled Method of Controlling the Film Thickness at a Blown Film Installation (hereafter, “the Upmeier patent”). The Upmeier patent discloses a method for providing a more uniform thickness distribution for a tubular film as is generally desired. The disclosed method, and system, uses microprocessors and sensors to determine control command for sectors of cooling or heating.
Although extensive efforts have been directed to achieve more uniform thickness in blown film extrusion, in order to realize certain product characteristics there have also been some efforts made to intentionally disrupt gauge uniformity. U.S. Pat. No. 6,139,186 issued to Robert W. Fraser and is entitled Bag Having Improved Tie Features (hereafter, “the Fraser patent”). The Fraser patent discloses a film formed by blown film extrusion with increased air cooling from the annular air ring where additional thickness is desired. Specifically, the Fraser patent discloses increased cooling in the blown film to provide thicker localized areas which can be generated by profiling the annular air ring opening at the areas where the additional thickness is desired.
However, the method disclosed by the Fraser patent is not ideal for all situations. The increased air cooling described by the Fraser patent requires permanent modification or profiling of the annular air ring. Thus, the resulting annular air ring of the Fraser patent is only suitable for use in the manufacture of tubular plastic film with the same gauge characteristics and profile. In addition to the foregoing limitations, the permanent modification of the annular air ring does not allow for controlling processing parameters due to changes in ambient manufacturing conditions such as temperature or humidity.
Another method known in the art for intentionally introducing gauge variations in tubular plastic film is to provide a secondary, external source of air to supplement the cooling provided by the annular air ring. For example, an additional air nozzle might be directed onto the bubble to provide additional cooling capacity. While the additional air nozzle may be utilized to provide such gauge variations in certain situations, the method also has certain disadvantages. The introduction of additional air along the cooling bubble can significantly impact the air flow around the bubble, the relative air pressure around the bubble, and can unpredictably change the flow of air around the cooling bubble of plastic film. As a result, bubble stability can be adversely affected and, by extension, the properties of the resultant film can be negatively impacted as well.
In view of the foregoing, it would be desirable to provide a method of providing controlled variations of film thickness in a plastic tubular film that does not significantly impact the air flow and air pressures around the plastic tubular film. It would also be desirable to provide a method for allowing for controlled cooling of the plastic film tube to induce thickness variation that does not require permanent modification of the manufacturing equipment, can be adjusted for different film properties or changes in ambient manufacturing conditions, and that can be quickly and easily enabled and disabled. The present invention addresses these needs.